High-nitrogen ferritic heat-resisting steel with high niobium content and method of production thereof

ABSTRACT

A high-nitrogen ferritic heat-resisting steel with high niobium content comprises, in weight per cent, 0.01-0.30% C, 0.02-0.80% Si, 0.20-1.00% Mn, 8.00-13.00% Cr, 0.005-1.00% Mo, 0.20-1.50% W, 0.05-1.00% V, over 0.12 up to 2.00% Nb and 0.10-0.50%N, the balance being Fe and unavoidable impurities. A method of producing the steel comprises melting and equilibrating the steel components in an atmosphere of a mixed gas of a prescribed nitrogen partial pressure or nitrogen gas and thereafter casting or solidifying the resulting melt in an atmosphere controlled to have a total pressure of not less than 2.5 bar and a nitrogen partial pressure of not less than 1.0 bar, with the relationship between the nitrogen partial pressure p and the total pressure P being 
     
         P&gt;2.5p.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a ferritic heat-resisting steel, more particularly to a high-nitrogen ferritic heat-resisting steel containing chromium, and being appropriate for use in a high-temperature, high-pressure environment, and to a method of producing the same.

2. Description of the Prior Art

Recent years have seen a marked increase in the temperatures and pressures under which thermal power plant boilers are required to operate. Some plans already call for operation at 566° C. and 314 bar and it is expected that operation under conditions of 650° C. and 355 bar will be implemented in the future. These are extremely severe conditions from the viewpoint of the boiler materials used.

At operating temperatures exceeding 550° C., it has, from the viewpoints of oxidation resistance and high-temperature strength, been necessary to switch from ferritic 2·1/4 Cr-1 Mo steel to high-grade austenitic steels such as 18-8 stainless steel. In other words, it has been necessary to adopt expensive materials with properties exceeding what is required.

Decades have been spent in search of steels for filling in the gap between 2·1/4 Cr-1 Mo steel and austenitic stainless steel. Medium Cr (e.g. 9 Cr and 12 Cr) steel boiler pipes are made of heat-resisting steels that were developed against this backdrop. They achieve high-temperature strength and creep rupture strength on a par with austenitic steels by use of a base metal composition which includes various alloying elements for precipitation hardening and solution hardening.

The creep rupture strength of a heat-resisting steel is governed by solution hardening in the case of short-term aging and by precipitation hardening in the case of prolonged aging. This is because the solution-hardening elements initially present in solid solution in the steel for the most part precipitate as stable carbides such as M₂₃ C₆ during aging, and then when the aging is prolonged these precipitates coagulate and enlarge, with a resulting decrease in creep rupture strength.

Thus, with the aim of maintaining the creep rupture strength of heat-resisting steels at a high level, a considerable amount of research has been done for discovering ways for avoiding the precipitation of the solution hardening elements and maintaining them in solid solution for as long as possible.

For example, Japanese Patent Public Disclosures No. Sho 63-89644, Sho 61-231139 and Sho 62-297435 teach ferritic steels that achieve dramatically higher creep rupture strength than conventional Mo-containing ferritic heat-resisting steels by the use of W as a solution hardening element.

While the solution hardening by W in these steels may be more effective than by Mo, the precipitates are still fundamentally carbides of the M₂₃ C₆ type, so that it is not possible to avoid reduction of the creep rupture strength with prolonged aging.

Moreover, the use of ferritic heat-resisting steels at up to 650° C. has been considered difficult because of their inferior high-temperature oxidation resistance as compared with austenitic heat-resisting steels. A particular problem with these steels is the pronounced degradation of high-temperature oxidation resistance that results from the precipitation of Cr in the form of coarse M₂₃ C₆ type precipitates at the grain boundaries.

The highest temperature limit for use of ferritic heat-resisting steel has therefore been considered to be 600° C.

The need for heat-resisting steels capable of standing up under extremely severe conditions has grown more acute not only because of the increasingly severe operating conditions mentioned earlier but also because of plans to reduce operating costs by extending the period of continuous power plant operation from the current 100 thousand hours up to around 150 thousand hours.

Although ferritic heat-resisting steels are somewhat inferior to austenitic steels in high-temperature strength and anticorrosion property, they have a cost advantage. Furthermore, for reasons related to the difference in thermal expansion coefficient, among the various steam oxidation resistance properties they are particularly superior in scale defoliation resistance. For these reasons, they are attracting attention as a boiler material.

For the reasons set out above, however, it is clearly not possible with the currently available technology to develop ferritic heat-resisting steels that are capable of standing up for 150 thousand hours under operating conditions of 650° C. and 355 bar, that are low in price and that exhibit good steam oxidation resistance.

Based on the foregoing knowledge and as described in Japanese Patent Application No. Hei 2-37895, the inventors earlier disclosed that a high-nitrogen ferritic heat-resisting steel estimated by linear extrapolation to exhibit a creep rupture strength of not less than 147 MPa under operating conditions of 650° C. and 355 bar for 150 thousand hours can be obtained by using a pressurized atmosphere to add nitrogen exceeding the solution limit and thus inducing precipitation of the excess nitrogen in the form of fine nitrides and carbo-nitrides. The gist of their disclosure was a ferritic heat-resisting steel characterized in comprising, in weight per cent, 0.01-0.30% C, 0.02-0.80% Si, 0.20-1.00% Mn, 8.00-13.00% Cr, 0.50-3.00% W, 0.005-1.00% Mo, 0.05-0.50% V, 0.02-0.12% Nb and 0.10-0.50%N and being controlled to include not more than 0.050% P, not more than 0.010% S and not more than 0.020% O, and optionally comprising (A) one or both of 0.01-1.00% Ta and 0.01-1.00% Hf and/or (B) one or both of 0.0005-0.10% Zr and 0.01-0.10% Ti, the balance being Fe and unavoidable impurities, and a method of producing the steel wherein the steel components are melted and equilibrated in an atmosphere of a mixed gas of a prescribed nitrogen partial pressure or nitrogen gas, and the resulting melt is thereafter cast or solidified in an atmosphere controlled to have a nitrogen partial pressure of not less than 1.0 bar and a total pressure of not less than 4.0 bar, with the relationship between the partial pressure p and the total pressure P being

    10.sup.p <P.sup.0.37 +log.sub.10.sup.6

thereby obtaining good quality ingot free of blowholes.

Based on the results of tests for determining the creep rupture strength of the steel taught by Japanese Patent Application No. Hei 2-37895 up to 50 thousand hours, the inventors discovered that the creep rupture strength of the steel at 150 thousand hours, as estimated by linear extrapolation, is no more than 176 MPa and, in particular, that the steel experiences a marked decrease in creep rupture strength between 30 and 50 thousand hours. Further studies showed that the reason for the decrease in creep rupture strength was that during the creep test large Fe₂ W grains measuring 1 μm or more in diameter precipitated in large amounts, principally at the grain boundaries, leading to large-scale loss of W as a solid solution element from the steel.

Moreover, they further discovered that by limiting the W content to not more than 1.5% so as to prevent precipitation of W as Fe₂ W and by adding Nb in excess of 0.12% so that NbN and (Nb, V)N, the most stable of all nitrides, become the principal precipitation hardening factors, it is possible to obtain a ferritic heat-resisting steel exhibiting a creep rupture strength at 650° C., 355 bar and 150 thousand hours of not less than 200 MPa, as estimated by linear extrapolation.

In addition they discovered that owing to the increase in the N solution limit resulting from the addition of a large amount of Nb the pressurized atmosphere conditions required for casting of sound ingot become a total pressure of not less than 2.5 bar and a nitrogen partial pressure of not less than 1.0 bar, with the relationship between the total pressure P and the nitrogen partial pressure p being

    P>2.5p.

There have been few papers published on research into high-nitrogen ferritic heat-resisting steels and the only known published report in this field in Ergebnisse der Werkstoff-Forschung, Band I, Varlag Schweizerische Akademie der Werkstoffwissenschaften "Thubal-Kain", Zurich, 1987, 161-180.

However, the research described in this report is limited to that in connection with ordinary heat-resisting steel and there is no mention of materials which can be used under such severe conditions at 650° C., 355 bar and 150 thousand hours continuous operation.

SUMMARY OF THE INVENTION

An object of this invention is to provide a high-nitrogen ferritic heat-resisting steel which overcomes the shortcomings of the conventional heat-resisting steels, and particularly to provide such a steel exhibiting outstanding creep rupture strength and capable of being used under severe operating conditions, wherein the decrease in creep rupture strength following prolonged aging and the degradation of high-temperature oxidation resistance caused by precipitation of carbides are mitigated by adding nitrogen to supersaturation so as to precipitate fine nitrides and/or carbo-nitrides which suppress the formation of carbides such as the M₂₃ C₆ precipitates seen in conventional steels.

This invention was accomplished in the light of the aforesaid knowledge and, in one aspect, pertains substantially to a high-nitrogen ferritic heat-resisting steel with high niobium content comprising, in weight percent 0.01-0.30% C, 0.02-0.80% Si, 0.20-1.00% Mn, 8.00-13.00% Cr, 0.005-1.00% Mo, 0.20-1.50% W, 0.05-1.00% V, over 0.12 up to 2.00% Nb and 0.10-0.50% N and being controlled to include not more than 0.050% P, not more than 0.010% S and not more than 0.020% O, and optionally comprising (A) one or both of 0.01-1.00% Ta and 0.01-1.00% Hf and/or (B) one or both of 0.0005-0.10% Zr and 0.01-0.10% Ti, the balance being Fe and unavoidable impurities.

Another aspect of the invention pertains to a method of producing such a high-nitrogen ferritic heat-resisting steel with high niobium content wherein the steel components are melted and equilibrated in an atmosphere of a mixed gas of a prescribed nitrogen partial pressure or nitrogen gas, and the resulting melt is thereafter cast or solidified in an atmosphere controlled to have a total pressure of not less than 2.5 bar and a nitrogen partial pressure of not less than 1.0 bar, with the relationship between the nitrogen partial pressure p and the total pressure P being

    P>2.5p.

The above and other features of the present invention will become apparent from the following description made with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an ingot and the manner in which it is to be cut.

FIG. 2 is a graph showing the relationship between the steel nitrogen content and the weight percentage of the total of M₂₃ C₆ +M₆ C+NbC+Cr₂ N+NbN among the precipitates in the steel accounted for by M₂₃ C₆ +M₆ C+NbC and the relationship between the steel nitrogen content and the weight percentage of the total of M₂₃ C₆ +M₆ C+NbC+Cr₂ N+NbN among the precipitates in the steel accounted for by Cr₂ N+NbN.

FIG. 3 is a graph showing conditions under which blowholes occur in the ingot in terms of the relationship between the total pressure and nitrogen partial pressure of the atmosphere during casting.

FIG. 4 is a schematic view showing the manner in which creep test pieces are taken from a pipe specimen and a rolled plate specimen.

FIG. 5 is a graph showing the relationship between steel nitrogen content and estimated creep rupture strength at 650° C., 150 thousand hours.

FIG. 6 is a graph showing the relationship between steel Nb content and estimated creep rupture strength at 650° C., 150 thousand hours.

FIG. 7 is a graph showing the relationship between steel W content and estimated creep rupture strength at 650° C., 150 thousand hours.

FIG. 8 is a graph showing an example of creep test results in terms of stress vs rupture time.

FIG. 9 is a graph showing the relationship between steel nitrogen content and Charpy impact absorption energy at 0° C. following aging at 700° C. for 10 thousand hours.

FIG. 10 is a graph showing the relationship between steel nitrogen content and the thickness of the oxidation scale formed on the surface of a test piece after oxidation at 650° C. for 10 thousand hours.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The reasons for the limits placed on the components of the high-nitrogen ferritic heat-resisting steel with high Nb content according to this invention will now be explained.

C is required for achieving strength. Adequate strength cannot be achieved at a C content of less than 0.01%, while at a C content exceeding 0.30% the steel is strongly affected by welding heat and undergoes hardening which becomes a cause for low-temperature cracking. The C content range is therefore set at 0.01-0.30%.

Si is important for achieving oxidation resistance and is also required as a deoxidizing agent. It is insufficient for these purposes at a content of less than 0.02%, whereas a content exceeding 0.80% reduces the creep rupture strength. The Si content range is therefore set at 0.02-0.80%.

Mn is required for deoxidation and also for achieving strength. It has to be added an amount of at least 0.20% for adequately exhibiting its effect. When it exceeds 1.00% it may in some cases reduce creep rupture strength. The Mn content range is therefore set at 0.20-1.00%.

Cr is indispensable to oxidation resistance. It also contributes to increasing creep resistance by combining with N and finely precipitating in the base metal matrix in the form of Cr₂ N, Cr₂ (C, N) and the like. Its lower limit is set at 8.00% from the viewpoint of oxidation resistance. Its upper limit is set at 13.00% for maintaining the Cr equivalent value at a low level so as to realize a martensite phase texture.

W produces a marked increase in creep rupture strength by solution hardening. Its effect toward increasing creep rupture strength over long periods at high temperatures of 550° C. and higher is particularly pronounced. Its upper limit is set at 1.50% because at contents higher than this level it precipitates in large quantities in the form of carbide and intermetallic compounds which sharply reduce the toughness of the base metal. The lower limit is set at 0.20% because it does not exhibit adequate solution hardening effect at lower levels.

Mo increases high-temperature strength through solution hardening. It does not exhibit adequate effect at a content of less than 0.005% and at a content higher than 1.00% it may, when added together with W, cause heavy precipitation of Mo₂ C type oxides which markedly reduce base metal toughness. The Mo content range is therefore set at 0.005-1.00%.

V produces a marked increase in the high-temperature strength of the steel regardless of whether it forms precipitates or, like W, enters solid solution in the matrix. When it precipitates, the resulting VN and (Nb, V)N serve as precipitation nuclei for Cr₂ N and Cr₂ (C, N), which has a pronounced effect toward promoting fine dispersion of the precipitates. It has no effect at below 0.05% and reduces toughness at higher than 1.00%. The V content range is therefore set at 0.05-1.00%.

Nb is an element which increases high-temperature strength by precipitating as NbN, (Nb, V)N, Nb(C, N) and (Nb, V) (C, N). Also, similarly to V, it promotes fine precipitate dispersion by forming precipitation nuclei for Cr₂ N, Cr₂ (C, N) and the like. For it to disperse in the steel as the primary precipitation hardening factor it has to be added in excess of 0.12%. However, its upper limit is set at 2.00% because when present at higher levels it reduces strength by causing precipitate coagulation and enlargement.

N dissolves in the matrix and also forms nitride and carbo-nitride precipitates. As the form of the precipitates is mainly Cr₂ N and Cr₂ (C, N), there is less precipitate-induced consumption of Cr and W than in the case of the M₂₃ C₆, M₆ C and other such precipitates observed in conventional steels. N thus increases oxidation resistance and creep rupture strength. At least 0.10% is required for precipitation of nitrides and carbo-nitrides and suppressing precipitation of M₂₃ C₆ and M₆ C. The upper limit is set at 0.50% for preventing coagulation and enlargement of nitride and carbo-nitride precipitates by the presence of excessive nitrogen.

P, S and O are present in the steel according to this invention as impurities. P and S hinder the achievement of the purpose of the invention by lowering strength, while O has the adverse effect of forming oxides which reduce toughness. The upper limits on these elements is therefore set at 0.050%, 0.010% and 0.020%, respectively.

The basic components of the steel according to this invention (aside from Fe) are as set out above. Depending on the purpose to which the steel is to be put, however, it may additionally contain (A) one or both of 0.01-1.00% Ta and 0.01-1.00% Hf and/or (B) one or both of 0.0005-0.10% Zr and 0.01-0.10% Ti.

At low concentrations Ta and Hf act as deoxidizing agents. At high concentrations they form fine high melting point nitrides and carbo-nitrides and, as such, increase toughness by decreasing the austenite grain size. In addition, they also reduce the degree to which Cr and W dissolve in precipitates and by this effect enhance the effect of supersaturation with nitrogen. Neither element exhibits any effect at less than 0.01%. When either is present at greater than 1.00%, it reduces toughness by causing enlargement of nitride and carbo-nitride precipitates. The content range of each of these elements is therefore set at 0.01-1.00%.

Acting to govern the deoxidation equilibrium in the steel, Zr suppresses the formation of oxides by markedly reducing the amount of oxygen activity. In addition, its strong affinity for N promotes precipitation of fine nitrides and carbo-nitrides which increase creep rupture strength and high-temperature oxidation resistance. When present at less than 0.0005% it does not provide an adequate effect of governing the deoxidation equilibrium and when present at greater than 0.10% it results in heavy precipitation of coarse ZrN and ZrC which markedly reduce the toughness of the base metal. The Zr content range is therefore set at 0.0005-0.10%.

Ti raises the effect of excess nitrogen by precipitating in the form of nitrides and carbo-nitrides. At a content of less than 0.01% it has no effect while a Ti content of over 0.10% results in precipitation of coarse nitrides and carbo-nitrides which reduce toughness. The Ti content range is therefore set at 0.01-0.10%.

The aforesaid alloying components can be added individually or in combinations.

The object of this invention is to provide a ferritic heat-resisting steel that is superior in creep rupture strength and high-temperature oxidation resistance. Depending on the purpose of use it can be produced by various methods and be subjected to various types of heat treatment. These methods and treatments in no way diminish the effect of the invention.

However, in view of the need to supersaturate the steel with nitrogen, it is necessary during casting to raise the total pressure of the atmosphere to not less than 2.5 bar and to control the relationship between the total pressure P and the nitrogen partial pressure p to satisfy the inequation P>2.5 p. As an auxiliary gas to be mixed with the nitrogen gas it is appropriate to use an inert gas such as Ar, Ne, Xe or Kr. These casting conditions were determined by the following experiment.

Steel of a chemical composition, aside from nitrogen, as indicated in the present invention was melted in an induction heating furnace installed in a chamber that could be pressurized up to 150 bar. A mixed gas of argon and nitrogen having a nitrogen partial pressure adequate for achieving the target nitrogen content was introduced into the furnace and maintained at a pressure which was varied from test to test. After the nitrogen and molten metal had reached chemical equilibrium, the molten metal was cast into a mold that had been installed in the chamber beforehand, whereby there was obtained a 5-ton ingot.

The ingot was cut vertically as shown in FIG. 1 and the ingot 1 was visually examined for the presence of blowholes.

Following this examination, a part of the ingot was placed in a furnace and maintained at 1180° C. for 1 hour and then forged into a plate measuring 50 mm in thickness, 750 mm in width and 4,000 mm in length.

This plate was subjected to solution treatment at 1200° C. for 1 hour and to tempering at 800° C. for 3 hours. The steel was then chemically analyzed and the dispersion state and morphology of the nitrides and carbo-nitrides were investigated by observation with an optical microscope, an electron microscope, X-ray diffraction and electron beam diffraction, whereby the chemical structure was determined.

Among the precipitates present within the as-heat-treated steel, FIG. 2 shows how the proportion of the precipitates in the steel accounted for by M₂₃ C₆ type carbides and M₆ C or NbC type carbides and the proportion thereof accounted for by Cr₂ N type nitrides and NbN type nitrides vary with nitrogen concentration. At a nitrogen concentration of 0.10%, nitrides account for the majority of the precipitates in the steel of the invention, while at a nitrogen concentration of 0.15%, substantially 100% of the precipitates are nitrides with virtually no carbides present whatsoever. Thus for the effect of this invention to be adequately manifested it is necessary for the nitrogen concentration of the steel to be not less than 0.1%.

The graph of FIG. 3 shows how the state of blowhole occurrence varies depending on the relationship between the total and nitrogen partial pressures of the atmosphere. For achieving a nitrogen concentration of 0.10% or higher it is necessary to use a total pressure of not less than 2.5 bar. Equilibrium calculation based on Sievert's law shows that in this case the nitrogen partial pressure in the steel of this invention is not less than 1.0 bar.

Moreover, where for controlling the amount of nitride and carbo-nitride precipitation the nitrogen partial pressure is maintained at 1.0-6.0 bar (nitrogen concentration within the steel of approximately 0.5 mass %), it becomes necessary to vary the total pressure between 2.5 and about 15 bar, the actual value selected depending on the nitrogen partial pressure. Namely, it is necessary to use a total pressure falling above the broken line representing the boundary pressure in FIG. 3.

When the boundary line of FIG. 3 is determined experimentally it is found to lie at

    P=2.5p

meaning that the steel according to this invention can be obtained by selecting an atmosphere of a pressure and composition meeting the condition of the inequality

    P>2.5p

It is therefore necessary to use furnace equipment enabling pressure and atmosphere control. Without such equipment, it is difficult to produce the steel of the present invention.

There are no limitations whatever on the melting method. Based on the chemical composition of the steel and cost considerations, it suffices to select from among processes using a converter, an induction heating furnace, an arc melting furnace or an electric furnace.

The situation regarding refining is similar. Insofar as the atmosphere is controlled to a total pressure of not less than 2.5 bar and a nitrogen partial pressure of not less than 1.0 bar, it is both possible and effective to use a ladle furnace, an electro-slag remelting furnace or a zone melting furnace.

After casting under a pressurized atmosphere of a total pressure of not less than 2.5 bar and a nitrogen partial pressure of not less than 1.0 bar, it is possible to process the steel into billet, bloom or plate by forging or hot rolling. Since the steel of this invention includes finely dispersed nitrides and carbo-nitrides, it is superior to conventional ferritic heat-resisting steels in hot-workability. This is also one reason for employing nitrides and carbo-nitrides obtained by adding nitrogen to beyond the solution limit.

For processing the steel into products, it is possible to first process it into round or rectangular billet and then form it into seamless pipe or tube by hot extrusion or any of various seamless rolling methods. Otherwise it can be formed into sheet by hot and cold rolling and then made into welded tube by electric resistance welding. Alternatively, it can be processed into welded pipe or tube by use of TIG, MIG, SAW, LASER and EB welding, individually or in combination. Moreover, it is possible to expand the size range of products to which the present invention can be applied by following any of the aforesaid processes by hot or warm stretch reduction or sizing.

The steel according to the invention can also be provided in the form of plate or sheet. The plate or sheet can, in its hot-rolled state or after whatever heat treatment is found necessary, be provided as a heat-resisting material in various shapes, without any influence on the effects provided by the invention.

The pipe, tube, plate, sheet and variously shaped heat-resisting materials referred to above can, in accordance with their purpose and application, be subjected to various heat treatments, and it is important for them to be so treated for realizing the full effect of the invention.

While the production process ordinarily involves normalizing (solution heat treatment)+ tempering, it is also possible and useful additionally to carry out one or a combination of two or more of quenching, tempering and normalizing. It is also possible, without influencing the effects of the present invention in any way, to repeatedly carry out one or more of the aforesaid processes to whatever degree is necessary for adequately bringing out the steel properties.

The aforesaid processes can be appropriately selected and applied to the manufacture of the steel according to the invention.

WORKING EXAMPLES

The steels indicated in Tables 1-14, each having a composition according to the present invention, were separately melted in amounts of 5 tons each in an induction heating furnace provided with pressurizing equipment. The resulting melt was cleaned by ladle furnace processing (under bubbling with a gas of the same composition as the atmosphere) for reducing its impurity content, whereafter the atmosphere was regulated using a mixed gas of nitrogen and argon so as to satisfy the conditions of the inequality P>2.5 p. The melt was then cast into a mold and processed into a round billet, part of which was hot extruded to obtain a tube 60 mm in outside diameter and 10 mm in wall thickness and the remainder of which was subjected to seamless rolling to obtain a pipe 380 mm in outside diameter and 50 mm in wall thickness. The tube and pipe were subjected to a single normalization at 1200° C. for 1 hour and were then tempered at 800° C. for 3 hours.

In addition, a 5 ton ingot was cast and forged into a slab which was hot rolled into 25 mm and 50 mm thick plates.

As shown in FIG. 4, creep test pieces 6 measuring 6 mm in diameter were taken along the axial direction 4 of the pipe or tube 3 and along the rolling direction 5 of the plates and subjected to creep test measurement at 650° C. Based on the data obtained, a linear extrapolation was made for estimating the creep rupture strength at 150 thousand hours. A creep rupture strength of 150 MPa was used as the creep rupture strength evaluation reference value. The creep rupture strength at 650° C., 150 thousand hours is hereinafter defined as the linearly extrapolated value at 150 thousand hours on the creep rupture strength vs rupture time graph.

Toughness was evaluated through an accelerated evaluation test in which aging was carried out at 700° C. for 10 thousand hours. JIS No. 4 tension test pieces were cut from the aged steel and evaluated for impact absorption energy. Assuming a water pressure test at 0° C., the toughness evaluation reference value was set at 10 J.

High-temperature oxidation resistance was evaluated by suspending a 25 mm×25 mm×5 mm test piece cut from the steel in 650° C. atmospheric air in a furnace for 10 thousand hours and then cutting the test piece parallel to the direction of growth of the scale and measuring the oxidation scale thickness.

The 650° C., 150 thousand hour creep rupture strength, the Charpy impact absorption energy at 0° C. after aging at 700° C. for 10 thousand hours and the oxidation scale thickness after oxidation at 650° C. for 10 thousand hours are shown in Tables 2, 4, 6, 8, 10, 12, and 14.

For comparison, steels of compositions not falling within the present invention were melted, processed and tested in the same way as described above. Their chemical compositions and the evaluation results are shown in Tables 15 and 16.

FIG. 5 shows the relationship between the nitrogen content of the steels and the estimated creep rupture strength at 650° C., 150 thousand hours. It will be noted that the creep rupture strength attains high values exceeding 150 MPa at a steel nitrogen content of 0.1% or higher but falls below 150 MPa and fails to satisfy the evaluation reference value that was set at a steel nitrogen content of less than 0.1%.

FIG. 6 shows the relationship between the Nb content of the steels and the estimated creep rupture strength at 650° C., 150 thousand hours. It will be noted that the creep rupture strength attains values exceeding 150 MPa at a steel Nb content exceeding 0.12% but at a Nb content of 2.0% or higher the creep rupture strength is instead lowered owing to the precipitation of coarse NbN and Fe₂ Nb type Laves phase at the melting stage.

FIG. 7 shows the relationship between the W content of the steels and the estimated creep rupture strength at 650° C., 150 thousand hours. The creep rupture strength is below 150 MPa at a W content of less than 0.2% and is 150 MPa or higher in a content range of 0.2-1.5%. When the W is present in excess of 1.5%, the creep rupture strength falls below 150 MPa owing to coarse Fe₂ W precipitating at the grain boundaries.

FIG. 8 shows the results of the creep test in terms of stress vs rupture time. A good linear relationship can be noted between stress and rupture time at a steel nitrogen content of not less than 0.1%. Moreover, the creep rupture strength is high. On the other hand, when the steel nitrogen content falls below 0.1%, the relationship between stress and rupture time exhibits a pronounced decline in creep rupture strength with increasing time lapse. Either the linearity is not maintained, or the slope of the creep rupture curve is steep, with the short-term side creep rupture strength being high but the long-term creep rupture strength being low, or the creep rupture strength is low throughout. This is because W and the other solution hardening elements precipitate as carbides whose coagulation and enlargement degrades the creep rupture strength property of the base metal. In contrast, at a nitrogen content of 0.1% or higher, fine nitrides are preferentially precipitated so that the formation of carbides is greatly delayed. Therefore, since the dissolution of the solution hardening elements into carbides was suppressed and also because the finely precipitated nitrides remained present in a stable state without coagulating and enlarging during the long-term high-temperature creep test, a high creep rupture strength was maintained in the long-term creep test.

FIG. 9 shows the relationship between Charpy impact absorption energy at 0° C. following aging at 700° C. for 10 thousand hours and steel nitrogen content. When the steel nitrogen content falls within the range of 0.1-0.5%, the impact absorption energy exceeds 10 J. In contrast, when it falls below 0.1%, there is little or no suppression of grain growth by residual high melting point nitrides during solution treatment and, as a result, the impact absorption energy decreases, and when it exceeds 0.5%, the impact absorption energy is reduced by heavy nitride precipitation.

FIG. 10 shows the relationship between the thickness of the oxidation scale formed on the surface of a test piece after oxidation at 650° C. for 10 thousand hours and the steel nitrogen content. Although the oxidation scale thickness is between 400 and 900 μm when the steel nitrogen content falls below 0.1%, it decreases to 50 μm or less when the steel nitrogen content is 0.1% or higher.

Reference is now made to the comparison steels shown in Table 5. Nos. 161 and 162 are examples in which insufficient steel nitrogen content resulted in a low estimated creep rupture strength at 650° C., 150 thousand hours and also to poor high-temperature oxidation resistance. Nos. 163 and 164 are examples in which excessive steel nitrogen content caused heavy precipitation of coarse nitrides and carbo-nitrides, resulting in a Charpy impact absorption energy at 0° C. after aging at 700° C. for 10 thousand hours of not more than 10 J. No. 165 is an example in which a low W concentration resulted in a low creep rupture strength at 650° C., 150 thousand hours owing to insufficient solution hardening notwithstanding that the steel nitrogen content fell within the range of the invention. No. 166 is an example in which a high W concentration led to low rupture strength and toughness owing to precipitation of coarse Fe₂ W type Laves phase at the grain boundaries during creep. No. 167 is an example in which a low Nb content resulted in a low estimated creep rupture strength at 650° C., 150 thousand hours. No. 168 is an example in which a high Nb content caused profuse precipitation of coarse Fe₂ Nb type Laves phase during creep, which in turn lowered both the estimated creep rupture strength at 650° C., 150 thousand hours and the Charpy impact absorption energy at 0° C. after aging at 700° C. for 10 thousand hours. No. 169 is an example in which heavy precipitation of coarse ZrN caused by a Zr concentration in excess of 0.1% resulted in a Charpy impact absorption energy at 0° C. after aging at 700° C. for 10 thousand hours of less than 10 J. Nos. 170, 171 and 172 are examples similar to the case of No. 169 except that the elements present in excess were Ta, Hf and Ti, respectively. As a result, heavy precipitation of coarse TaN, HfN and TiN resulted in a Charpy impact absorption energy at 0° C. after aging at 700° C. for 10 thousand hours of less than 10 J. No. 173 is an example in which, notwithstanding that the steel composition satisfied the conditions of the present invention, since the nitrogen partial pressure was 2.2 bar and the total pressure was 2.5 bar, values not satisfying the inequality P>2.5 p, many large blowholes formed in the ingot, making it impossible to obtain either a sound ingot or a plate and leading to a reduction in both the estimated creep rupture strength at 650° C., 150 thousand hours and the Charpy impact absorption energy at 0° C. after aging at 700° C. for 10 thousand hours.

                  TABLE 1                                                          ______________________________________                                         (mass %) Invention steels                                                      No.  C      Si     Mn   P    S     Nb   V    Cr   Mo                           ______________________________________                                          1   0.195  0.251  0.801                                                                               0.048                                                                               0.008 0.666                                                                               0.094                                                                                9.52                                                                               0.586                         2   0.070  0.475  0.632                                                                               0.013                                                                               0.005 0.132                                                                               0.370                                                                               10.38                                                                               0.038                         3   0.192  0.709  0.261                                                                               0.042                                                                               0.009 1.185                                                                               0.174                                                                                8.70                                                                               0.707                         4   0.032  0.316  0.231                                                                               0.022                                                                               0.008 0.418                                                                               0.487                                                                               12.80                                                                               0.767                         5   0.219  0.220  0.299                                                                               0.018                                                                               0.004 1.064                                                                               0.496                                                                               11.18                                                                               0.427                         6   0.096  0.190  0.620                                                                               0.022                                                                               0.003 0.608                                                                               0.066                                                                                9.82                                                                               0.865                         7   0.288  0.762  0.546                                                                               0.038                                                                               0.008 1.308                                                                               0.402                                                                                9.66                                                                               0.283                         8   0.270  0.515  0.216                                                                               0.014                                                                               0.001 0.680                                                                               0.462                                                                                9.07                                                                               0.843                         9   0.098  0.369  0.583                                                                               0.039                                                                               0.003 0.927                                                                               0.105                                                                               11.51                                                                               0.748                        10   0.259  0.477  0.995                                                                               0.039                                                                               0.002 0.582                                                                               0.089                                                                               10.40                                                                               0.179                        11   0.083  0.594  0.404                                                                               0.021                                                                               0.007 1.824                                                                               0.050                                                                               11.70                                                                               0.820                        12   0.206  0.478  0.961                                                                               0.035                                                                               0.004 0.336                                                                               0.073                                                                                8.84                                                                               0.366                        13   0.100  0.663  0.812                                                                               0.026                                                                               0.003 1.163                                                                               0.469                                                                               12.74                                                                               0.504                        14   0.113  0.515  0.514                                                                               0.043                                                                               0.004 1.837                                                                               0.418                                                                               11.87                                                                               0.416                        15   0.152  0.577  0.217                                                                               0.033                                                                               0.007 0.823                                                                               0.237                                                                               12.21                                                                               0.804                        16   0.127  0.600  0.467                                                                               0.024                                                                               0.002 0.392                                                                               0.454                                                                                9.62                                                                               0.843                        17   0.092  0.722  0.203                                                                               0.022                                                                               0.006 1.747                                                                               0.246                                                                               12.49                                                                               0.704                        18   0.243  0.320  0.395                                                                               0.036                                                                               0.007 1.583                                                                               0.165                                                                                9.38                                                                               0.613                        19   0.060  0.713  0.388                                                                               0.013                                                                               0.001 0.194                                                                               0.285                                                                               11.20                                                                               0.732                        20   0.030  0.087  0.722                                                                               0.020                                                                               0.009 0.790                                                                               0.465                                                                                8.83                                                                               0.751                        21   0.210  0.544  0.354                                                                               0.026                                                                               0.004 1.791                                                                               0.341                                                                               10.73                                                                               0.363                        22   0.213  0.281  0.237                                                                               0.036                                                                               0.008 1.714                                                                               0.451                                                                               10.38                                                                               0.756                        23   0.101  0.227  0.520                                                                               0.041                                                                               0.008 0.897                                                                               0.200                                                                                8.70                                                                               0.117                        24   0.067  0.128  0.544                                                                               0.045                                                                               0.001 0.640                                                                               0.348                                                                                9.08                                                                               0.625                        25   0.172  0.142  0.473                                                                               0.038                                                                               0.002 0.340                                                                               0.307                                                                               12.53                                                                               0.482                        ______________________________________                                    

                  TABLE 2                                                          ______________________________________                                                                                   CS   VE   TO                         No.  W      Zr     Ta   Hf  Ti  N    O    MPa  J    μm                      ______________________________________                                          1   1.154  --     --   --  --  0.375                                                                               0.002                                                                               231  19.4 46                          2   0.716  --     --   --  --  0.242                                                                               0.010                                                                               214  17.7 46                          3   1.169  --     --   --  --  0.422                                                                               0.019                                                                               230  55.3 38                          4   1.439  --     --   --  --  0.187                                                                               0.013                                                                               195  38.7 28                          5   0.497  --     --   --  --  0.210                                                                               0.007                                                                               183  54.3 47                          6   1.187  --     --   --  --  0.376                                                                               0.002                                                                               209  77.9 19                          7   0.496  --     --   --  --  0.153                                                                               0.012                                                                               243  18.8 13                          8   0.608  --     --   --  --  0.217                                                                               0.012                                                                               153  64.1 36                          9   0.473  --     --   --  --  0.440                                                                               0.006                                                                               157  16.9 36                         10   0.329  --     --   --  --  0.301                                                                               0.017                                                                               226  31.2 20                         11   0.420  0.050  --   --  --  0.270                                                                               0.009                                                                               226  14.1 16                         12   0.999  0.092  --   --  --  0.427                                                                               0.006                                                                               178  49.6 11                         13   1.006  0.031  --   --  --  0.445                                                                               0.019                                                                               201  45.7 33                         14   1.328  0.035  --   --  --  0.197                                                                               0.016                                                                               161  15.1 50                         15   0.280  0.073  --   --  --  0.164                                                                               0.010                                                                               179  41.0 27                         16   0.686  0.015  --   --  --  0.218                                                                               0.012                                                                               175  32.4 35                         17   1.464  0.026  --   --  --  0.228                                                                               0.017                                                                               213  35.1 38                         18   0.323  0.028  --   --  --  0.276                                                                               0.002                                                                               166  64.9 46                         19   1.363  0.040  --   --  --  0.438                                                                               0.005                                                                               202  28.6 26                         20   0.280  0.080  --   --  --  0.183                                                                               0.013                                                                               240  45.7 26                         21   0.574  --     0.291                                                                               --  --  0.190                                                                               0.019                                                                               246  56.8 32                         22   1.196  --     0.414                                                                               --  --  0.134                                                                               0.018                                                                               242  42.0 12                         23   1.086  --     0.275                                                                               --  --  0.293                                                                               0.013                                                                               195  13.6 34                         24   0.480  --     0.038                                                                               --  --  0.132                                                                               0.017                                                                               226  55.8 25                         25   0.926  --     0.039                                                                               --  --  0.139                                                                               0.006                                                                               200  75.2 13                         ______________________________________                                          CS: Creep rupture strength at 650° C., 150 thousand hours;              VE: Charpy impact absorption energy at 0° C. after aging at             700° C. for 10 thousand hours;                                          TO: Oxidation scale thickness after 650° C., 10 thousand hour           hightemperature oxidation                                                

                  TABLE 3                                                          ______________________________________                                         (mass %) Invention steels                                                      No.  C      Si     Mn   P    S     Nb   V    Cr   Mo                           ______________________________________                                         26   0.027  0.796  0.445                                                                               0.045                                                                               0.005 1.306                                                                               0.157                                                                               12.42                                                                               0.859                        27   0.273  0.684  0.616                                                                               0.036                                                                               0.006 1.054                                                                               0.209                                                                               12.23                                                                               0.954                        28   0.093  0.727  0.740                                                                               0.018                                                                               0.007 1.824                                                                               0.100                                                                               10.41                                                                               0.459                        29   0.120  0.290  0.555                                                                               0.017                                                                               0.004 1.164                                                                               0.363                                                                               10.34                                                                               0.055                        30   0.118  0.298  0.215                                                                               0.023                                                                               0.006 0.328                                                                               0.177                                                                               10.47                                                                               0.914                        31   0.144  0.520  0.282                                                                               0.041                                                                               0.008 0.648                                                                               0.087                                                                                9.32                                                                               0.756                        32   0.248  0.694  0.487                                                                               0.019                                                                               0.007 0.572                                                                               0.391                                                                               10.20                                                                               0.241                        33   0.055  0.208  0.602                                                                               0.046                                                                               0.001 1.432                                                                               0.286                                                                                9.19                                                                               0.080                        34   0.198  0.752  0.361                                                                               0.048                                                                               0.002 1.631                                                                               0.296                                                                               12.13                                                                               0.939                        35   0.292  0.519  0.709                                                                               0.013                                                                               0.005 1.583                                                                               0.056                                                                                9.91                                                                               0.011                        36   0.106  0.449  0.217                                                                               0.049                                                                               0.003 1.617                                                                               0.334                                                                               11.12                                                                               0.494                        37   0.127  0.685  0.219                                                                               0.023                                                                               0.002 0.190                                                                               0.304                                                                               11.57                                                                               0.563                        38   0.118  0.576  0.459                                                                               0.022                                                                               0.006 0.492                                                                               0.278                                                                                9.73                                                                               0.406                        39   0.279  0.286  0.703                                                                               0.014                                                                               0.001 1.016                                                                               0.344                                                                               11.55                                                                               0.220                        40   0.237  0.746  0.301                                                                               0.027                                                                               0.003 1.137                                                                               0.138                                                                                8.01                                                                               0.691                        41   0.036  0.137  0.987                                                                               0.028                                                                               0.004 0.275                                                                               0.408                                                                               12.45                                                                               0.993                        42   0.024  0.558  0.524                                                                               0.023                                                                               0.001 1.619                                                                               0.351                                                                               10.85                                                                               0.824                        43   0.111  0.602  0.658                                                                               0.043                                                                               0.006 1.759                                                                               0.272                                                                               11.70                                                                               0.878                        44   0.063  0.416  0.740                                                                               0.013                                                                               0.007 1.035                                                                               0.252                                                                               10.85                                                                               0.331                        45   0.220  0.528  0.523                                                                               0.040                                                                               0.009 0.723                                                                               0.380                                                                                9.33                                                                               0.379                        46   0.203  0.303  0.544                                                                               0.013                                                                               0.010 0.266                                                                               0.060                                                                               10.46                                                                               0.195                        47   0.211  0.280  0.956                                                                               0.039                                                                               0.006 0.957                                                                               0.055                                                                               12.74                                                                               0.470                        48   0.158  0.227  0.728                                                                               0.025                                                                               0.005 0.334                                                                               0.227                                                                                9.64                                                                               0.408                        49   0.241  0.279  0.798                                                                               0.020                                                                               0.004 1.557                                                                               0.210                                                                               12.12                                                                               0.195                        50   0.078  0.197  0.915                                                                               0.030                                                                               0.008 0.262                                                                               0.310                                                                                8.54                                                                               0.524                        ______________________________________                                    

                                      TABLE 4                                      __________________________________________________________________________     (mass %) Invention steels                                                                                  CS  VE TO                                          No. W  Zr  Ta Hf  Ti N   O  MPa J  μm                                       __________________________________________________________________________     26  1.374                                                                             --  0.611                                                                             --  -- 0.377                                                                              0.001                                                                             238 15.8                                                                              45                                          27  0.756                                                                             --  0.221                                                                             --  -- 0.329                                                                              0.016                                                                             160 30.8                                                                              49                                          28  1.272                                                                             --  0.295                                                                             --  -- 0.468                                                                              0.007                                                                             232 63.1                                                                              13                                          29  1.225                                                                             --  0.134                                                                             --  -- 0.266                                                                              0.015                                                                             225 55.9                                                                              23                                          30  0.834                                                                             --  0.227                                                                             --  -- 0.199                                                                              0.007                                                                             156 21.9                                                                              27                                          31  1.018                                                                             0.017                                                                              0.053                                                                             --  -- 0.149                                                                              0.013                                                                             194 79.6                                                                              33                                          32  1.356                                                                             0.007                                                                              0.337                                                                             --  -- 0.379                                                                              0.019                                                                             159 32.3                                                                              29                                          33  1.460                                                                             0.012                                                                              0.494                                                                             --  -- 0.378                                                                              0.019                                                                             221 68.5                                                                              21                                          34  0.246                                                                             0.003                                                                              0.098                                                                             --  -- 0.411                                                                              0.016                                                                             194 59.9                                                                              37                                          35  1.227                                                                             0.017                                                                              0.325                                                                             --  -- 0.353                                                                              0.005                                                                             200 21.0                                                                              26                                          36  1.401                                                                             0.001                                                                              0.118                                                                             --  -- 0.284                                                                              0.017                                                                             158 69.4                                                                              13                                          37  0.337                                                                             0.084                                                                              0.681                                                                             --  -- 0.476                                                                              0.009                                                                             186 77.7                                                                              47                                          38  0.899                                                                             0.031                                                                              0.079                                                                             --  -- 0.399                                                                              0.002                                                                             152 24.2                                                                              15                                          39  0.726                                                                             0.013                                                                              0.549                                                                             --  -- 0.135                                                                              0.019                                                                             176 28.7                                                                              26                                          40  0.722                                                                             0.046                                                                              0.062                                                                             --  -- 0.163                                                                              0.009                                                                             227 32.3                                                                              45                                          41  0.450                                                                             --  -- 0.844                                                                              -- 0.148                                                                              0.001                                                                             171 48.4                                                                              41                                          42  1.484                                                                             --  -- 0.732                                                                              -- 0.168                                                                              0.010                                                                             180 55.9                                                                              42                                          43  1.463                                                                             --  -- 0.509                                                                              -- 0.301                                                                              0.019                                                                             172 67.8                                                                              14                                          44  1.266                                                                             --  -- 0.36                                                                               -- 0.423                                                                              0.014                                                                             165 26.7                                                                              46                                          45  0.495                                                                             --  -- 0.954                                                                              -- 0.202                                                                              0.007                                                                             226 50.2                                                                              43                                          46  0.695                                                                             --  -- 0.516                                                                              -- 0.116                                                                              0.002                                                                             227 47.5                                                                              35                                          47  1.456                                                                             --  -- 0.215                                                                              -- 0.439                                                                              0.012                                                                             235 66.8                                                                              49                                          48  0.429                                                                             --  -- 0.239                                                                              -- 0.254                                                                              0.013                                                                             221 16.1                                                                              33                                          49  1.116                                                                             --  -- 0.775                                                                              -- 0.229                                                                              0.007                                                                             152 48.8                                                                              13                                          50  1.143                                                                             --  -- 0.511                                                                              -- 0.241                                                                              0.008                                                                             195 71.6                                                                              18                                          __________________________________________________________________________      CS: Creep rupture strength at 650° C., 150 thousand hours;              VE: Charpy impact absorption energy at 0° C. after aging at             700° C. for 10 thousand hours;                                          TO: Oxidation scale thickness after 650° C., 10 thousand hour           hightemperature oxidation                                                

                  TABLE 5                                                          ______________________________________                                         (mass %) Invention steels                                                      No.  C      Si     Mn   P    S     Nb   V    Cr   Mo                           ______________________________________                                         51   0.013  0.514  0.430                                                                               0.043                                                                               0.001 0.562                                                                               0.385                                                                                9.86                                                                               0.024                        52   0.244  0.202  0.686                                                                               0.027                                                                               0.008 1.922                                                                               0.074                                                                                9.51                                                                               0.451                        53   0.251  0.594  0.652                                                                               0.029                                                                               0.003 0.881                                                                               0.135                                                                               10.99                                                                               0.969                        54   0.229  0.256  0.857                                                                               0.036                                                                               0.002 0.636                                                                               0.203                                                                               10.17                                                                               0.274                        55   0.091  0.135  0.271                                                                               0.016                                                                               0.001 0.590                                                                               0.138                                                                                8.04                                                                               0.047                        56   0.032  0.745  0.624                                                                               0.017                                                                               0.007 1.097                                                                               0.471                                                                               12.99                                                                               0.466                        57   0.252  0.293  0.474                                                                               0.026                                                                               0.007 1.554                                                                               0.160                                                                                8.45                                                                               0.225                        58   0.081  0.484  0.769                                                                               0.030                                                                               0.009 1.403                                                                               0.416                                                                               12.86                                                                               0.661                        59   0.011  0.299  0.870                                                                               0.018                                                                               0.008 0.988                                                                               0.430                                                                               12.23                                                                               0.017                        60   0.093  0.653  0.510                                                                               0.015                                                                               0.004 0.628                                                                               0.051                                                                                9.89                                                                               0.914                        61   0.236  0.532  0.650                                                                               0.031                                                                               0.006 1.657                                                                               0.267                                                                               11.83                                                                               0.408                        62   0.207  0.470  0.638                                                                               0.032                                                                               0.001 0.660                                                                               0.493                                                                               11.53                                                                               0.187                        63   0.060  0.620  0.630                                                                               0.026                                                                               0.007 1.736                                                                               0.134                                                                                9.91                                                                               0.926                        64   0.138  0.327  0.757                                                                               0.021                                                                               0.005 0.544                                                                               0.396                                                                               12.80                                                                               0.318                        65   0.061  0.155  0.791                                                                               0.024                                                                               0.004 1.198                                                                               0.245                                                                               11.28                                                                               0.057                        66   0.175  0.617  0.869                                                                               0.022                                                                               0.008 0.296                                                                               0.436                                                                               11.21                                                                               0.317                        67   0.095  0.311  0.345                                                                               0.023                                                                               0.005 1.922                                                                               0.377                                                                               11.03                                                                               0.788                        68   0.169  0.165  0.971                                                                               0.015                                                                               0.007 0.839                                                                               0.384                                                                                9.23                                                                               0.673                        69   0.018  0.714  0.898                                                                               0.045                                                                               0.002 0.192                                                                               0.314                                                                                9.23                                                                               0.154                        70   0.271  0.143  0.664                                                                               0.020                                                                               0.004 0.309                                                                               0.471                                                                                9.42                                                                               0.402                        71   0.119  0.553  0.823                                                                               0.029                                                                               0.003 0.583                                                                               0.261                                                                                8.20                                                                               0.605                        72   0.033  0.126  0.712                                                                               0.036                                                                               0.003 0.506                                                                               0.447                                                                                8.99                                                                               0.944                        73   0.240  0.227  0.929                                                                               0.013                                                                               0.003 0.416                                                                               0.228                                                                               10.50                                                                               0.658                        74   0.054  0.575  0.388                                                                               0.045                                                                               0.009 1.583                                                                               0.129                                                                               10.93                                                                               0.898                        75   0.116  0.159  0.450                                                                               0.041                                                                               0.009 1.533                                                                               0.233                                                                                8.81                                                                               0.361                        ______________________________________                                    

                                      TABLE 6                                      __________________________________________________________________________     (mass %) Invention steels                                                                                  CS  VE TO                                          No. W  Zr  Ta Hf  Ti N   O  MPa J  μm                                       __________________________________________________________________________     51  0.750                                                                             0.065                                                                              -- 0.132                                                                              -- 0.363                                                                              0.010                                                                             171 28.4                                                                              13                                          52  0.200                                                                             0.047                                                                              -- 0.829                                                                              -- 0.469                                                                              0.007                                                                             249 38.4                                                                              49                                          53  0.378                                                                             0.073                                                                              -- 0.297                                                                              -- 0.243                                                                              0.011                                                                             247 78.5                                                                              31                                          54  0.882                                                                             0.066                                                                              -- 0.709                                                                              -- 0.348                                                                              0.009                                                                             234 57.7                                                                              48                                          55  1.307                                                                             0.043                                                                              -- 0.767                                                                              -- 0.271                                                                              0.002                                                                             178 23.3                                                                              26                                          56  0.583                                                                             0.022                                                                              -- 0.647                                                                              -- 0.277                                                                              0.004                                                                             199 23.7                                                                              14                                          57  0.908                                                                             0.072                                                                              -- 0.033                                                                              -- 0.242                                                                              0.010                                                                             213 36.2                                                                              18                                          58  0.573                                                                             0.064                                                                              -- 0.619                                                                              -- 0.444                                                                              0.003                                                                             243 40.8                                                                              47                                          59  1.144                                                                             0.064                                                                              -- 0.603                                                                              -- 0.359                                                                              0.016                                                                             205 14.2                                                                              27                                          60  0.883                                                                             0.042                                                                              -- 0.150                                                                              -- 0.257                                                                              0.015                                                                             217 39.2                                                                              46                                          61  0.699                                                                             --  0.207                                                                             0.955                                                                              -- 0.218                                                                              0.012                                                                             200 25.1                                                                              33                                          62  1.497                                                                             --  0.536                                                                             0.229                                                                              -- 0.117                                                                              0.002                                                                             222 19.8                                                                              33                                          63  0.258                                                                             --  0.341                                                                             0.135                                                                              -- 0.227                                                                              0.007                                                                             201 35.9                                                                              16                                          64  0.468                                                                             --  0.360                                                                             0.355                                                                              -- 0.220                                                                              0.002                                                                             207 51.8                                                                              27                                          65  0.441                                                                             --  0.396                                                                             0.595                                                                              -- 0.377                                                                              0.009                                                                             157 10.9                                                                              15                                          66  0.405                                                                             --  0.897                                                                             0.534                                                                              -- 0.168                                                                              0.004                                                                             172 30.7                                                                              48                                          67  1.254                                                                             --  0.097                                                                             0.849                                                                              -- 0.272                                                                              0.010                                                                             167 54.5                                                                              12                                          68  0.646                                                                             --  0.016                                                                             0.264                                                                              -- 0.363                                                                              0.001                                                                             166 61.3                                                                              16                                          69  1.421                                                                             --  0.143                                                                             0.434                                                                              -- 0.318                                                                              0.019                                                                             184 28.0                                                                              26                                          70  1.206                                                                             --  0.178                                                                             0.542                                                                              -- 0.404                                                                              0.001                                                                             191 73.5                                                                              11                                          71  0.881                                                                             0.045                                                                              0.012                                                                             0.273                                                                              -- 0.301                                                                              0.017                                                                             202 32.9                                                                              15                                          72  1.365                                                                             0.019                                                                              0.047                                                                             0.337                                                                              -- 0.272                                                                              0.004                                                                             242 29.6                                                                              11                                          73  0.980                                                                             0.015                                                                              0.162                                                                             0.924                                                                              -- 0.223                                                                              0.013                                                                             227 74.0                                                                              43                                          74  0.592                                                                             0.051                                                                              0.010                                                                             0.719                                                                              -- 0.404                                                                              0.019                                                                             227 75.1                                                                              20                                          75  1.100                                                                             0.046                                                                              0.178                                                                             0.898                                                                              -- 0.101                                                                              0.018                                                                             209 79.9                                                                              28                                          __________________________________________________________________________      CS: Creep rupture strength at 650° C., 150 thousand hours;              VE: Charpy impact absorption energy at 0° C. after aging at             700° C. for 10 thousand hours;                                          TO: Oxidation scale thickness after 650° C., 10 thousand hour           hightemperature oxidation                                                

                  TABLE 76                                                         ______________________________________                                         (mass %) Invention steels                                                      No.  C      Si     Mn   P    S     Nb   V    Cr   Mo                           ______________________________________                                         76   0.038  0.278  0.848                                                                               0.046                                                                               0.008 1.558                                                                               0.167                                                                               12.46                                                                               0.708                        77   0.168  0.707  0.952                                                                               0.030                                                                               0.009 1.092                                                                               0.253                                                                               10.45                                                                               0.161                        78   0.271  0.415  0.904                                                                               0.020                                                                               0.005 0.466                                                                               0.344                                                                               11.15                                                                               0.886                        79   0.253  0.712  0.448                                                                               0.022                                                                               0.010 0.980                                                                               0.296                                                                               12.97                                                                               0.574                        80   0.027  0.196  0.545                                                                               0.045                                                                               0.003 1.114                                                                               0.203                                                                               11.93                                                                               0.423                        81   0.228  0.180  0.543                                                                               0.036                                                                               0.006 1.228                                                                               0.370                                                                               11.44                                                                               0.614                        82   0.184  0.028  0.988                                                                               0.022                                                                               0.005 0.696                                                                               0.428                                                                               12.41                                                                               0.834                        83   0.092  0.657  0.819                                                                               0.012                                                                               0.006 1.776                                                                               0.235                                                                                8.22                                                                               0.755                        84   0.296  0.277  0.206                                                                               0.031                                                                               0.003 1.997                                                                               0.107                                                                               12.43                                                                               0.591                        85   0.275  0.590  0.894                                                                               0.049                                                                               0.006 1.727                                                                               0.127                                                                               10.93                                                                               0.521                        86   0.082  0.182  0.627                                                                               0.013                                                                               0.009 1.291                                                                               0.375                                                                                9.62                                                                               0.223                        87   0.201  0.362  0.750                                                                               0.049                                                                               0.006 1.062                                                                               0.487                                                                                9.47                                                                               0.886                        88   0.116  0.513  0.228                                                                               0.026                                                                               0.002 0.437                                                                               0.326                                                                                8.93                                                                               0.710                        89   0.161  0.761  0.800                                                                               0.011                                                                               0.002 0.717                                                                               0.195                                                                               11.38                                                                               0.866                        90   0.254  0.099  0.223                                                                               0.031                                                                               0.003 0.383                                                                               0.187                                                                               12.86                                                                               0.066                        91   0.299  0.227  0.243                                                                               0.015                                                                               0.002 0.718                                                                               0.155                                                                               11.73                                                                               0.969                        92   0.063  0.509  0.608                                                                               0.049                                                                               0.001 0.188                                                                               0.184                                                                                8.92                                                                               0.777                        93   0.179  0.315  0.673                                                                               0.020                                                                               0.006 1.041                                                                               0.360                                                                               10.41                                                                               0.680                        94   0.269  0.573  0.589                                                                               0.018                                                                               0.006 0.808                                                                               0.317                                                                               12.14                                                                               0.863                        95   0.239  0.643  0.497                                                                               0.048                                                                               0.006 0.746                                                                               0.137                                                                               12.49                                                                               0.502                        96   0.142  0.665  0.549                                                                               0.019                                                                               0.008 1.266                                                                               0.357                                                                                9.91                                                                               0.796                        97   0.182  0.791  0.892                                                                               0.018                                                                               0.005 1.684                                                                               0.152                                                                               12.81                                                                               0.418                        98   0.260  0.786  0.889                                                                               0.031                                                                               0.008 0.605                                                                               0.300                                                                               12.25                                                                               0.083                        99   0.298  0.405  0.687                                                                               0.049                                                                               0.009 0.167                                                                               0.197                                                                               11.60                                                                               0.461                        100  0.198  0.566  0.429                                                                               0.017                                                                               0.001 1.715                                                                               0.383                                                                               11.77                                                                               0.241                        ______________________________________                                    

                                      TABLE 8                                      __________________________________________________________________________     (mass %) Invention steels                                                                                  CS  VE TO                                          No. W  Zr  Ta Hf  Ti N   O  MPa J  μm                                       __________________________________________________________________________     76  0.770                                                                             0.069                                                                              0.562                                                                             0.644                                                                              -- 0.178                                                                              0.011                                                                             242 32.0                                                                              36                                          77  1.183                                                                             0.039                                                                              0.104                                                                             0.666                                                                              -- 0.321                                                                              0.002                                                                             195 14.8                                                                              30                                          78  1.261                                                                             0.024                                                                              0.157                                                                             0.136                                                                              -- 0.200                                                                              0.014                                                                             187 40.8                                                                              19                                          79  1.250                                                                             0.042                                                                              0.071                                                                             0.233                                                                              -- 0.487                                                                              0.013                                                                             235 53.2                                                                              49                                          80  0.762                                                                             0.065                                                                              0.019                                                                             0.660                                                                              -- 0.464                                                                              0.003                                                                             212 31.2                                                                              29                                          81  0.676                                                                             --  -- --  0.082                                                                             0.204                                                                              0.014                                                                             183 57.2                                                                              26                                          82  0.581                                                                             --  -- --  0.081                                                                             0.380                                                                              0.013                                                                             167 38.9                                                                              46                                          83  0.716                                                                             --  -- --  0.090                                                                             0.264                                                                              0.004                                                                             184 59.2                                                                              38                                          84  0.695                                                                             --  -- --  0.083                                                                             0.134                                                                              0.018                                                                             202 62.3                                                                              10                                          85  1.407                                                                             --  -- --  0.076                                                                             0.268                                                                              0.001                                                                             241 61.1                                                                              29                                          86  1.129                                                                             --  -- --  0.084                                                                             0.137                                                                              0.007                                                                             218 67.7                                                                              21                                          87  1.415                                                                             --  -- --  0.082                                                                             0.415                                                                              0.017                                                                             152 61.7                                                                              16                                          88  1.211                                                                             --  -- --  0.040                                                                             0.183                                                                              0.008                                                                             213 20.0                                                                              19                                          89  0.758                                                                             --  -- --  0.065                                                                             0.309                                                                              0.012                                                                             168 26.2                                                                              40                                          90  0.956                                                                             --  -- --  0.039                                                                             0.473                                                                              0.004                                                                             238 58.4                                                                              24                                          91  1.400                                                                             0.002                                                                              -- --  0.069                                                                             0.349                                                                              0.006                                                                             190 60.0                                                                              41                                          92  1.017                                                                             0.024                                                                              -- --  0.037                                                                             0.210                                                                              0.002                                                                             243 79.0                                                                              16                                          93  1.367                                                                             0.039                                                                              -- --  0.050                                                                             0.133                                                                              0.016                                                                             217 58.3                                                                              39                                          94  0.736                                                                             0.020                                                                              -- --  0.080                                                                             0.195                                                                              0.014                                                                             179 46.5                                                                              36                                          95  1.317                                                                             0.083                                                                              -- --  0.052                                                                             0.466                                                                              0.016                                                                             198 55.2                                                                              34                                          96  1.405                                                                             0.051                                                                              -- --  0.075                                                                             0.218                                                                              0.010                                                                             194 64.5                                                                              41                                          97  0.565                                                                             0.037                                                                              -- --  0.073                                                                             0.132                                                                              0.015                                                                             232 49.8                                                                              45                                          98  0.412                                                                             0.066                                                                              -- --  0.023                                                                             0.482                                                                              0.009                                                                             237 28.1                                                                              14                                          99  0.479                                                                             0.004                                                                              -- --  0.027                                                                             0.347                                                                              0.003                                                                             223 45.0                                                                              34                                          100 0.394                                                                             0.069                                                                              -- --  0.016                                                                             0.396                                                                              0.001                                                                             162 79.7                                                                              14                                          __________________________________________________________________________      CS: Creep rupture strength at 650° C., 150 thousand hours;              VE: Charpy impact absorption energy at 0° C. after aging at             700° C. for 10 thousand hours;                                          TO: Oxidation scale thickness after 650° C., 10 thousand hour           hightemperature oxidation                                                

                  TABLE 9                                                          ______________________________________                                         (mass %) Invention steels                                                      No.  C      Si     Mn   P    S     Nb   V    Cr   Mo                           ______________________________________                                         101  0.019  0.256  0.274                                                                               0.023                                                                               0.004 0.876                                                                               0.266                                                                               11.27                                                                               0.616                        102  0.100  0.392  0.765                                                                               0.014                                                                               0.001 1.758                                                                               0.380                                                                               10.96                                                                               0.270                        103  0.061  0.732  0.450                                                                               0.038                                                                               0.008 1.892                                                                               0.357                                                                               11.79                                                                               0.383                        104  0.249  0.253  0.688                                                                               0.043                                                                               0.008 1.646                                                                               0.284                                                                                9.74                                                                               0.154                        105  0.282  0.275  0.824                                                                               0.036                                                                               0.007 1.357                                                                               0.470                                                                               10.74                                                                               0.763                        106  0.212  0.148  0.595                                                                               0.044                                                                               0.007 0.453                                                                               0.098                                                                                8.14                                                                               0.897                        107  0.099  0.043  0.977                                                                               0.015                                                                               0.006 0.226                                                                               0.380                                                                                8.02                                                                               0.757                        108  0.284  0.478  0.899                                                                               0.017                                                                               0.004 1.955                                                                               0.499                                                                                8.69                                                                               0.513                        109  0.237  0.048  0.922                                                                               0.017                                                                               0.004 1.291                                                                               0.312                                                                               11.17                                                                               0.856                        110  0.167  0.070  0.793                                                                               0.043                                                                               0.004 0.482                                                                               0.135                                                                               11.37                                                                               0.489                        111  0.123  0.680  0.262                                                                               0.021                                                                               0.003 1.753                                                                               0.495                                                                               12.72                                                                               0.084                        112  0.156  0.085  0.278                                                                               0.020                                                                               0.009 1.696                                                                               0.356                                                                               11.17                                                                               0.827                        113  0.038  0.770  0.994                                                                               0.047                                                                               0.002 0.663                                                                               0.475                                                                               10.15                                                                               0.416                        114  0.108  0.533  0.552                                                                               0.029                                                                               0.009 1.814                                                                               0.485                                                                               10.58                                                                               0.526                        115  0.274  0.302  0.304                                                                               0.014                                                                               0.005 1.769                                                                               0.473                                                                                8.42                                                                               0.851                        116  0.015  0.624  0.410                                                                               0.030                                                                               0.006 1.807                                                                               0.300                                                                               12.12                                                                               0.587                        117  0.255  0.467  0.614                                                                               0.024                                                                               0.003 0.392                                                                               0.272                                                                               10.10                                                                               0.910                        118  0.177  0.373  0.265                                                                               0.023                                                                               0.004 1.667                                                                               0.476                                                                                8.30                                                                               0.017                        119  0.272  0.483  0.721                                                                               0.036                                                                               0.002 1.254                                                                               0.323                                                                               10.19                                                                               0.590                        120  0.156  0.164  0.520                                                                               0.029                                                                               0.001 1.393                                                                               0.118                                                                                9.17                                                                               0.815                        121  0.030  0.730  0.614                                                                               0.030                                                                               0.005 1.490                                                                               0.395                                                                               11.15                                                                               0.877                        122  0.239  0.382  0.759                                                                               0.046                                                                               0.009 1.912                                                                               0.116                                                                                9.62                                                                               0.474                        123  0.111  0.044  0.701                                                                               0.026                                                                               0.007 1.124                                                                               0.489                                                                               10.50                                                                               0.281                        124  0.283  0.645  0.990                                                                               0.036                                                                               0.001 0.417                                                                               0.250                                                                               12.20                                                                               0.493                        125  0.226  0.762  0.575                                                                               0.024                                                                               0.005 0.131                                                                               0.157                                                                               10.70                                                                               0.298                        ______________________________________                                    

                                      TABLE 10                                     __________________________________________________________________________     (mass %) Invention steels                                                                                  CS  VE TO                                          No. W  Zr  Ta Hf  Ti N   O  MPa J  μm                                       __________________________________________________________________________     101 0.381                                                                             --  0.089                                                                             --  0.094                                                                             0.310                                                                              0.017                                                                             170 39.6                                                                              40                                          102 1.346                                                                             --  0.057                                                                             --  0.054                                                                             0.160                                                                              0.016                                                                             217 43.3                                                                              21                                          103 0.263                                                                             --  0.646                                                                             --  0.046                                                                             0.218                                                                              0.012                                                                             221 68.1                                                                              43                                          104 0.645                                                                             --  0.097                                                                             --  0.044                                                                             0.164                                                                              0.007                                                                             152 57.9                                                                              15                                          105 1.425                                                                             --  0.213                                                                             --  0.019                                                                             0.273                                                                              0.005                                                                             227 25.4                                                                              23                                          106 0.904                                                                             --  0.941                                                                             --  0.034                                                                             0.213                                                                              0.019                                                                             216 19.2                                                                              15                                          107 1.275                                                                             --  0.589                                                                             --  0.060                                                                             0.268                                                                              0.006                                                                             189 17.4                                                                              20                                          108 0.827                                                                             --  0.257                                                                             --  0.068                                                                             0.170                                                                              0.007                                                                             211 32.1                                                                              43                                          109 1.104                                                                             --  0.440                                                                             --  0.031                                                                             0.210                                                                              0.015                                                                             155 77.8                                                                              34                                          110 1.060                                                                             --  0.545                                                                             --  0.056                                                                             0.450                                                                              0.016                                                                             236 62.2                                                                              38                                          111 0.271                                                                             0.039                                                                              0.030                                                                             --  0.024                                                                             0.298                                                                              0.004                                                                             225 67.8                                                                              45                                          112 0.296                                                                             0.052                                                                              0.119                                                                             --  0.043                                                                             0.234                                                                              0.010                                                                             192 38.7                                                                              20                                          113 0.729                                                                             0.085                                                                              0.011                                                                             --  0.084                                                                             0.334                                                                              0.019                                                                             202 57.0                                                                              13                                          114 1.148                                                                             0.009                                                                              0.066                                                                             --  0.073                                                                             0.466                                                                              0.016                                                                             176 31.0                                                                              29                                          115 1.156                                                                             0.041                                                                              0.174                                                                             --  0.073                                                                             0.108                                                                              0.005                                                                             182 79.5                                                                              17                                          116 0.997                                                                             0.052                                                                              0.030                                                                             --  0.012                                                                             0.139                                                                              0.017                                                                             241 76.1                                                                              25                                          117 0.921                                                                             0.007                                                                              0.023                                                                             --  0.086                                                                             0.432                                                                              0.009                                                                             219 32.3                                                                              20                                          118 1.171                                                                             0.032                                                                              0.149                                                                             --  0.086                                                                             0.262                                                                              0.011                                                                             187 67.8                                                                              28                                          119 1.341                                                                             0.089                                                                              0.057                                                                             --  0.030                                                                             0.105                                                                              0.010                                                                             212 19.7                                                                              39                                          120 0.364                                                                             0.049                                                                              0.199                                                                             --  0.024                                                                             0.232                                                                              0.010                                                                             213 76.1                                                                              39                                          121 1.085                                                                             --  -- 0.518                                                                              0.085                                                                             0.321                                                                              0.002                                                                             158 40.8                                                                              35                                          122 0.507                                                                             --  -- 0.911                                                                              0.096                                                                             0.245                                                                              0.010                                                                             168 31.1                                                                              16                                          123 0.813                                                                             --  -- 0.693                                                                              0.072                                                                             0.482                                                                              0.010                                                                             161 61.2                                                                              49                                          124 0.958                                                                             --  -- 0.058                                                                              0.032                                                                             0.307                                                                              0.015                                                                             204 73.3                                                                              40                                          125 1.176                                                                             --  -- 0.171                                                                              0.035                                                                             0.389                                                                              0.018                                                                             217 45.3                                                                              39                                          __________________________________________________________________________      CS: Creep rupture strength at 650° C., 150 thousand hours;              VE: Charpy impact absorption energy at 0° C. after aging at             700° C. for 10 thousand hours;                                          TO: Oxidation scale thickness after 650° C., 10 thousand hour           hightemperature oxidation                                                

                  TABLE 11                                                         ______________________________________                                         (mass %) Invention steels                                                      No.  C      Si     Mn   P    S     Nb   V    Cr   Mo                           ______________________________________                                         126  0.283  0.545  0.806                                                                               0.037                                                                               0.009 0.633                                                                               0.067                                                                               11.89                                                                               0.372                        127  0.079  0.145  0.236                                                                               0.044                                                                               0.003 0.416                                                                               0.070                                                                               11.77                                                                               0.918                        128  0.101  0.490  0.613                                                                               0.011                                                                               0.009 0.723                                                                               0.246                                                                               11.22                                                                               0.617                        129  0.112  0.546  0.487                                                                               0.046                                                                               0.001 1.964                                                                               0.230                                                                               11.21                                                                               0.157                        130  0.168  0.078  0.806                                                                               0.016                                                                               0.007 0.672                                                                               0.102                                                                               10.14                                                                               0.713                        131  0.176  0.388  0.916                                                                               0.029                                                                               0.008 1.932                                                                               0.406                                                                               11.45                                                                               0.488                        132  0.048  0.090  0.586                                                                               0.013                                                                               0.008 0.474                                                                               0.325                                                                                8.67                                                                               0.178                        133  0.274  0.484  0.754                                                                               0.035                                                                               0.005 1.318                                                                               0.165                                                                               12.68                                                                               0.954                        134  0.180  0.543  0.766                                                                               0.042                                                                               0.006 1.076                                                                               0.457                                                                               11.88                                                                               0.494                        135  0.258  0.024  0.373                                                                               0.023                                                                               0.007 1.989                                                                               0.391                                                                                9.73                                                                               0.029                        136  0.130  0.645  0.536                                                                               0.044                                                                               0.006 0.611                                                                               0.278                                                                               12.22                                                                               0.208                        137  0.277  0.191  0.985                                                                               0.033                                                                               0.009 0.139                                                                               0.415                                                                               10.17                                                                               0.214                        138  0.181  0.384  0.681                                                                               0.017                                                                               0.004 1.865                                                                               0.224                                                                               12.09                                                                               0.745                        139  0.181  0.587  0.978                                                                               0.044                                                                               0.006 1.433                                                                               0.235                                                                                8.94                                                                               0.500                        140  0.044  0.160  0.419                                                                               0.022                                                                               0.003 0.142                                                                               0.082                                                                               11.61                                                                               0.652                        141  0.116  0.041  0.761                                                                               0.018                                                                               0.003 1.168                                                                               0.072                                                                               11.08                                                                               0.397                        142  0.015  0.763  0.554                                                                               0.019                                                                               0.001 1.115                                                                               0.076                                                                                8.29                                                                               0.928                        143  0.231  0.128  0.741                                                                               0.033                                                                               0.004 1.269                                                                               0.393                                                                                9.13                                                                               0.422                        144  0.237  0.626  0.679                                                                               0.028                                                                               0.005 1.969                                                                               0.434                                                                                9.59                                                                               0.072                        145  0.236  0.452  0.514                                                                               0.011                                                                               0.001 1.700                                                                               0.059                                                                                8.08                                                                               0.062                        146  0.135  0.148  0.803                                                                               0.030                                                                               0.007 1.742                                                                               0.254                                                                                8.08                                                                               0.083                        147  0.129  0.624  0.481                                                                               0.031                                                                               0.002 0.580                                                                               0.425                                                                               10.43                                                                               0.812                        148  0.279  0.092  0.512                                                                               0.044                                                                               0.007 0.434                                                                               0.209                                                                                8.66                                                                               0.454                        149  0.200  0.253  0.237                                                                               0.034                                                                               0.009 0.723                                                                               0.138                                                                               11.16                                                                               0.182                        150  0.237  0.466  0.610                                                                               0.049                                                                               0.009 1.793                                                                               0.296                                                                               11.37                                                                               0.184                        ______________________________________                                    

                                      TABLE 12                                     __________________________________________________________________________     (mass %) Invention steels                                                                                  CS  VE TO                                          No. W  Zr  Ta Hf  Ti N   O  MPa J  μm                                       __________________________________________________________________________     126 1.059                                                                             --  -- 0.733                                                                              0.010                                                                             0.207                                                                              0.004                                                                             219 60.8                                                                              11                                          127 0.872                                                                             --  -- 0.945                                                                              0.019                                                                             0.237                                                                              0.014                                                                             229 63.5                                                                              46                                          128 1.484                                                                             --  -- 0.472                                                                              0.033                                                                             0.156                                                                              0.015                                                                             214 30.4                                                                              49                                          129 1.198                                                                             --  -- 0.523                                                                              0.070                                                                             0.432                                                                              0.018                                                                             246 10.9                                                                              50                                          130 0.892                                                                             --  -- 0.752                                                                              0.055                                                                             0.346                                                                              0.001                                                                             190 51.7                                                                              34                                          131 1.487                                                                             0.008                                                                              -- 0.873                                                                              0.050                                                                             0.167                                                                              0.003                                                                             166 61.5                                                                              11                                          132 0.625                                                                             0.059                                                                              -- 0.091                                                                              0.075                                                                             0.180                                                                              0.018                                                                             245 19.4                                                                              47                                          133 0.490                                                                             0.011                                                                              -- 0.559                                                                              0.099                                                                             0.453                                                                              0.013                                                                             193 77.2                                                                              49                                          134 1.317                                                                             0.029                                                                              -- 0.965                                                                              0.049                                                                             0.263                                                                              0.011                                                                             230 73.7                                                                              26                                          135 1.413                                                                             0.081                                                                              -- 0.038                                                                              0.034                                                                             0.482                                                                              0.005                                                                             175 57.5                                                                              24                                          136 1.023                                                                             0.035                                                                              -- 0.412                                                                              0.068                                                                             0.392                                                                              0.007                                                                             195 75.0                                                                              32                                          137 0.370                                                                             0.012                                                                              -- 0.798                                                                              0.080                                                                             0.257                                                                              0.012                                                                             211 70.9                                                                              16                                          138 1.018                                                                             0.078                                                                              -- 0.887                                                                              0.040                                                                             0.360                                                                              0.015                                                                             240 22.6                                                                              25                                          139 0.601                                                                             0.053                                                                              -- 0.365                                                                              0.056                                                                             0.174                                                                              0.010                                                                             202 13.2                                                                              25                                          140 1.318                                                                             0.055                                                                              -- 0.912                                                                              0.010                                                                             0.373                                                                              0.018                                                                             214 55.7                                                                              24                                          141 1.461                                                                             --  0.893                                                                             0.227                                                                              0.099                                                                             0.433                                                                              0.005                                                                             197 59.3                                                                              30                                          142 1.291                                                                             --  0.389                                                                             0.493                                                                              0.025                                                                             0.233                                                                              0.005                                                                             219 36.6                                                                              31                                          143 1.031                                                                             --  0.277                                                                             0.404                                                                              0.029                                                                             0.442                                                                              0.010                                                                             199 70.7                                                                              14                                          144 0.928                                                                             --  0.128                                                                             0.139                                                                              0.039                                                                             0.339                                                                              0.013                                                                             155 69.5                                                                              45                                          145 0.579                                                                             --  0.089                                                                             0.880                                                                              0.072                                                                             0.199                                                                              0.003                                                                             206 11.7                                                                              34                                          146 0.875                                                                             --  0.348                                                                             0.329                                                                              0.098                                                                             0.297                                                                              0.017                                                                             221 49.5                                                                              27                                          147 1.417                                                                             --  0.113                                                                             0.662                                                                              0.045                                                                             0.100                                                                              0.013                                                                             168 21.4                                                                              11                                          148 1.142                                                                             --  0.247                                                                             0.075                                                                              0.020                                                                             0.494                                                                              0.004                                                                             217 10.9                                                                              15                                          149 0.288                                                                             --  0.022                                                                             0.243                                                                              0.040                                                                             0.149                                                                              0.013                                                                             185 64.9                                                                              25                                          150 0.401                                                                             --  0.372                                                                             0.357                                                                              0.076                                                                             0.337                                                                              0.005                                                                             242 58.3                                                                              10                                          __________________________________________________________________________      CS: Creep rupture strength at 650° C., 150 thousand hours;              VE: Charpy impact absorption energy at 0° C. after aging at             700° C. for 10 thousand hours;                                          TO: Oxidation scale thickness after 650° C., 10 thousand hour           hightemperature oxidation                                                

                  TABLE 13                                                         ______________________________________                                         (mass %) Invention steels                                                      No.  C      Si     Mn   P    S     Nb   V    Cr   Mo                           ______________________________________                                         151  0.157  0.497  0.978                                                                               0.037                                                                               0.010 0.234                                                                               0.321                                                                               12.72                                                                               0.652                        152  0.154  0.087  0.687                                                                               0.026                                                                               0.008 1.123                                                                               0.289                                                                                8.62                                                                               0.764                        153  0.144  0.189  0.303                                                                               0.023                                                                               0.001 1.021                                                                               0.072                                                                               10.12                                                                               0.231                        154  0.176  0.143  0.360                                                                               0.015                                                                               0.009 0.282                                                                               0.193                                                                                8.02                                                                               0.213                        155  0.159  0.104  0.608                                                                               0.010                                                                               0.002 1.169                                                                               0.288                                                                               10.81                                                                               0.401                        156  0.010  0.619  0.814                                                                               0.016                                                                               0.008 1.219                                                                               0.230                                                                                9.53                                                                               0.516                        157  0.069  0.270  0.320                                                                               0.020                                                                               0.007 0.757                                                                               0.317                                                                               10.02                                                                               0.178                        158  0.246  0.373  0.476                                                                               0.049                                                                               0.010 0.142                                                                               0.186                                                                               10.76                                                                               0.205                        159  0.159  0.680  0.631                                                                               0.032                                                                               0.008 0.675                                                                               0.143                                                                               10.13                                                                               0.649                        160  0.152  0.556  0.529                                                                               0.035                                                                               0.004 1.745                                                                               0.275                                                                                9.10                                                                               0.748                        ______________________________________                                    

                                      TABLE 14                                     __________________________________________________________________________     (mass %) Invention steels                                                                                  CS  VE TO                                          No. W  Zr  Ta Hf  Ti N   O  MPa J  μm                                       __________________________________________________________________________     151 0.429                                                                             0.091                                                                              0.090                                                                             0.113                                                                              0.055                                                                             0.338                                                                              0.002                                                                             206 37.3                                                                              16                                          152 0.610                                                                             0.090                                                                              0.715                                                                             0.686                                                                              0.085                                                                             0.421                                                                              0.017                                                                             197 51.9                                                                              12                                          153 0.554                                                                             0.012                                                                              0.131                                                                             0.772                                                                              0.091                                                                             0.269                                                                              0.017                                                                             183 43.9                                                                              24                                          154 0.478                                                                             0.071                                                                              0.497                                                                             0.062                                                                              0.047                                                                             0.350                                                                              0.014                                                                             231 26.6                                                                              40                                          155 1.283                                                                             0.091                                                                              0.492                                                                             0.726                                                                              0.013                                                                             0.340                                                                              0.003                                                                             212 13.8                                                                              29                                          156 1.301                                                                             0.090                                                                              0.202                                                                             0.490                                                                              0.053                                                                             0.120                                                                              0.005                                                                             245 53.5                                                                              16                                          157 0.741                                                                             0.085                                                                              0.361                                                                             0.510                                                                              0.048                                                                             0.164                                                                              0.015                                                                             199 19.2                                                                              31                                          158 1.158                                                                             0.046                                                                              0.140                                                                             0.980                                                                              0.015                                                                             0.147                                                                              0.010                                                                             242 28.4                                                                              20                                          159 0.833                                                                             0.091                                                                              0.089                                                                             0.591                                                                              0.015                                                                             0.339                                                                              0.009                                                                             192 24.2                                                                              31                                          160 0.668                                                                             0.038                                                                              0.034                                                                             0.026                                                                              0.020                                                                             0.218                                                                              0.008                                                                             218 54.3                                                                              10                                          __________________________________________________________________________      CS: Creep rupture strength at 650° C., 150 thousand hours;              VE: Charpy impact absorption energy at 0° C. after aging at             700° C. for 10 thousand hours;                                          TO: Oxidation scale thickness after 650° C., 10 thousand hour           hightemperature oxidation                                                

                                      TABLE 15                                     __________________________________________________________________________     Comparison steels (mass %)                                                     No. C  Si  Mn P   S  Nb  V  Cr  Mo W                                           __________________________________________________________________________     161 0.02                                                                              0.06                                                                               0.45                                                                              0.014                                                                              0.008                                                                             0.051                                                                              0.20                                                                              8.53                                                                               0.592                                                                             0.87                                        162 0.07                                                                              0.07                                                                               0.57                                                                              0.012                                                                              0.004                                                                             0.066                                                                              0.22                                                                              8.99                                                                               0.424                                                                             0.88                                        163 0.23                                                                              0.10                                                                               0.52                                                                              0.016                                                                              0.004                                                                             0.074                                                                              0.18                                                                              9.05                                                                               0.560                                                                             0.86                                        164 0.15                                                                              0.13                                                                               0.45                                                                              0.011                                                                              0.002                                                                             0.033                                                                              0.19                                                                              9.23                                                                               0.550                                                                             0.77                                        165 0.29                                                                              0.12                                                                               0.98                                                                              0.008                                                                              0.001                                                                             0.127                                                                              0.08                                                                              12.1                                                                               0.320                                                                             0.11                                        166 0.15                                                                              0.09                                                                               0.24                                                                              0.009                                                                              0.001                                                                             0.290                                                                              0.45                                                                              12.7                                                                               0.007                                                                             1.80                                        167 0.18                                                                              0.09                                                                               0.44                                                                              0.009                                                                              0.001                                                                             0.014                                                                              0.31                                                                              11.4                                                                               0.989                                                                             0.99                                        168 0.11                                                                              0.12                                                                               0.74                                                                              0.004                                                                              0.001                                                                             2.880                                                                              0.22                                                                              11.8                                                                               0.679                                                                             0.67                                        169 0.09                                                                              0.21                                                                               0.81                                                                              0.044                                                                              0.002                                                                             0.321                                                                              0.20                                                                              10.5                                                                               0.814                                                                             0.23                                        170 0.10                                                                              0.23                                                                               0.99                                                                              0.024                                                                              0.001                                                                             0.545                                                                              0.19                                                                              10.0                                                                               0.333                                                                             0.77                                        171 0.22                                                                              0.21                                                                               0.11                                                                              0.012                                                                              0.001                                                                             1.227                                                                              0.40                                                                              9.76                                                                               0.545                                                                             1.21                                        172 0.07                                                                              0.27                                                                               0.07                                                                              0.013                                                                              0.009                                                                             1.621                                                                              0.49                                                                              9.00                                                                               0.512                                                                             1.09                                        173 0.23                                                                              0.23                                                                               0.09                                                                              0.002                                                                              0.010                                                                             1.998                                                                              0.07                                                                              8.86                                                                               0.533                                                                             1.15                                        __________________________________________________________________________

                  TABLE 16                                                         ______________________________________                                         Comparison steels (mass %)                                                                                             CS   VE   TO                           No.  Zr     Ta     Hf   Ti   N     O    MPa  J    (μm)                      ______________________________________                                         161  0.007  0.65   --   0.034                                                                               0.072 0.007                                                                               120  70   760                          162  0.008  0.77   --   0.031                                                                               0.081 0.009                                                                               107  28   660                          163  0.002  0.78   --   0.044                                                                               0.872 0.007                                                                               205  6    50                           164  0.012  0.71   0.66 0.100                                                                               0.525 0.002                                                                               185  3    35                           165  0.011  0.76   0.87 0.010                                                                               0.164 0.002                                                                                65  95   35                           166  --     --     0.81 --   0.128 0.002                                                                                90  2    20                           167  --     --     0.59 --   0.154 0.001                                                                                70  60   25                           168  --     --     --   0.060                                                                               0.332 0.002                                                                               116  4     5                           169  0.145  0.89   --   0.071                                                                               0.425 0.002                                                                               187  4    25                           170  --     1.21   --   0.032                                                                               0.202 0.002                                                                               153  7    40                           171  0.011  0.32   1.13 --   0.191 0.008                                                                               220  7    45                           172  0.540  0.05   0.22 0.29 0.103 0.012                                                                               210  8    15                           173  0.880  --     0.10 --   0.200 0.006                                                                                24  2    30                           ______________________________________                                          CS: Creep rupture strength at 650° C., 150 thousand hours;              VE: Charpy impact absorption energy at 0° C. after aging at             700° C. for 10 thousand hours;                                          TO: Oxidation scale thickness after 650° C., 10 thousand hour           hightemperature oxidation                                                

The present invention provides a high-nitrogen ferritic heat-resisting steel with high Nb content exhibiting a high rupture strength after prolonged creep and superior high-temperature oxidation resistance and, as such, can be expected to make a major contribution to industrial progress. 

What is claimed is:
 1. A high-nitrogen ferritic heat-resisting steel with high Nb content, consisting of, in weight percent0.01-0.30% C, 0.02-0.80% Si, 0.20-1.00% Mn, 8.00-13.00% Cr, 0.005-1.00% Mo, 0.20-1.50% W, 0.05-1.00% V, over 0.12 up to 2.00% Nb, and 0.10-0.50% N,and being controlled to include not more than 0.050% P, not more than 0.010% S, and not more than 0.020% O,the remainder being Fe and unavoidable impurities.
 2. A high-nitrogen ferritic heat-resisting steel with high Nb content consisting of, in weight percent0.01-0.30% C, 0.02-0.80% Si, 0.20-1.00% Mn, 8.00-13.00% Cr, 0.005-1.00% Mo, 0.20-1.50% W, 0.05-1.00% V, over 0.12 up to 2.00% Nb, and
 0. 10-0.50% N,and one or both of 0.01-1.00% Ta and 0.01-1.00% Hfand being controlled to include not more than 0.050% P, not more than 0.010% S, and not more than 0.020% O,the remainder being Fe and unavoidable impurities.
 3. A high-nitrogen ferritic heat-resisting steel with high Nb content consisting of, in weight percent0.01-0.30% C, 0.02-0.80% Si, 0.20-1.00% Mn, 8.00-13.00% Cr, 0.005-1.00% Mo, 0.20-1.50% W, 0.05-1.00% V, over 0.12 up to 2.00% Nb, and 0.10-0.50% N,and one or both of 0.0005-0.10% Zr and 0.01-0.10% Tiand being controlled to include not more than 0.050% P, not more than 0.010% S, and not more than 0.020% O,the remainder being Fe and unavoidable impurities.
 4. A high-nitrogen ferritic heat-resisting steel with high Nb content consisting of, in weight percent0.01-0.30% C, 0.02-0.80% Si, 0.20-1.00% Mn, 8.00-13.00% Cr, 0.005-1.00% Mo, 0.20-1.50% W, 0.05-1.00% V, over 0.12 up to 2.00% Nb, and 0.10-0.50% N,one or both of 0.01-1.00% Ta and 0.01-1.00% Hfand one or both of 0.0005-0.10% Zr and 0.01-0.10% Tiand being controlled to include not more than 0.050% P, not more than 0.010% S, and not more than 0.020% O,the remainder being Fe and unavoidable impurities.
 5. A method of producing a high-nitrogen ferritic heat-resisting steel with high Nb content having a composition according to claim 1, wherein the steel is melted and equilibrated in an atmosphere of a mixed gas of a prescribed nitrogen partial pressure or nitrogen gas and is thereafter cast or solidified in an atmosphere controlled to have a total pressure of not less than 2.5 bar and a nitrogen partial pressure of not less than 1.0 bar, with the relationship between the nitrogen partial pressure p and the total pressure P being

    P>2.5p.


6. A method of producing a high-nitrogen ferritic heat-resisting steel with high Nb content having a composition according to claim 2, wherein the steel is melted and equilibrated in an atmosphere of a mixed gas of a prescribed nitrogen partial pressure or nitrogen gas and is thereafter cast or solidified in an atmosphere controlled to have a total pressure of not less than 2.5 bar and a nitrogen partial pressure of not less than 1.0 bar, with the relationship between the nitrogen partial pressure p and the total pressure P being

    P>2.5p.


7. A method of producing a high-nitrogen ferritic heat-resisting steel with high Nb content having a composition according to claim 3, wherein the steel is melted and equilibrated in an atmosphere of a mixed gas of a prescribed nitrogen partial pressure or nitrogen gas and is thereafter cast or solidified in an atmosphere controlled to have a total pressure of not less than 2.5 bar and a nitrogen partial pressure of not less than 1.0 bar, with the relationship between the nitrogen partial pressure p and the total pressure P being

    P>2.5p.


8. A method of producing a high-nitrogen ferritic heat-resisting steel with high Nb content having a composition according to claim 4, wherein the steel is melted and equilibrated in an atmosphere of a mixed gas of a prescribed nitrogen partial pressure or nitrogen gas and is thereafter cast or solidified in an atmosphere controlled to have a total pressure of not less than 2.5 bar and a nitrogen partial pressure of not less than 1.0 bar, with the relationship between the nitrogen partial pressure p and the total pressure P being

    P>2.5p. 